Add configurable outputWriters

GIT NOTE:"
I'm new to github, and I hope that this commit won't break anything - We had a few unrelated changes in our repository which would be automatically included in the pull request. The only way I got around that was to base my branch on an older version of the repository.


I've added the ability to specify outputWriters in the YAML:

outputWriters:
    - "@class": "com.googlecode.jmxtrans.model.output.GraphiteWriter"
      settings:
          host: "localhost"
          port: 2003

NOTES:
- I've attempted to be completely backwards compatible.
- PLEASE test this before running in production